The production of printed circuit boards by the so-called "build-up" method is known in the art. In this method, an insulating carrier or base material, e.g., a molded laminate, is provided with a relatively thin layer of copper on its surface. The surface of the copper layer is then printed with a layer of masking material, which covers all of the areas not corresponding to the desired pattern of conductor lines. The desired pattern of conductor lines is then built up, preferably by means of the galvanic or electrolytic deposition of copper, and if desired, additional metals such as tin/lead alloys, nickel, gold, and the like. Thereafter, the layer of masking material is removed and the thin layer of previously masked copper, which is now exposed, is decomposed by an etching treatment.
Printed circuit boards manufactured by the procedure just described, the conductor lines of which consist exclusively of copper, have penetrated numerous fields of application, for reasons of economy in manufacture and because of their excellent solderability. The introduction of the so-called "build-up" method has also made practical the use of printed circuits with metallized through-hole walls in the field of electronics. Before then, the inherently high costs of production precluded their use, despite acknowledged advantages in assembly of apparatus and in the improved reliability of soldered connections.
Such prior art methods are subject to a number of disadvantages, however. In general, in these methods the surface of the carrier material, which preferably is provided with an adhesion-promoting layer, the latter being made microporous using known techniques, is catalyzed for the electroless deposition of metal using a sensitizing solution containing palladium, and preferably a solution of a Pd (II)/Sn (II) chloride complex. Then, using known procedures, a thin layer of copper having a thickness, for example, of from 1 to 3 microns, is built up by means of electroless deposition.
Preferably, for the production of printed circuits having metallized through-hole walls, perforations are made in the carrier material before the application of the metal layer by electroless deposition. After electroless deposition, a masking material is applied in the known manner, e.g., by means of screen printing or photographing printing, and the unmasked, i.e., exposed, areas of the electrolessly deposited copper layer are then built up to the thickness desired for the conductor lines by means of electrolytic deposition, the copper layer acting as the current supply.
After the layer of masking material is removed, this portion of the electrolessly deposited layer of copper, now exposed, must be removed by etching. In order to ensure adequately high surface resistance values between adjacent conductor lines, it is necessary not only to completely remove the electrolessly deposited copper, but also any residues from the catalyzing step. On the other hand, especially in the case of conductor lines built up with copper only, care must be taken to prevent too great an amount of the conductor line copper from being carried away in the etching treatment. Such excess losses of copper, especially in the case of metallized through-hole walls, result in the formation of defects in the layer of deposited copper, unless the etching treatment is very carefully controlled.
It is known that longer etching times are made possible by electrolytically depositing a layer of copper of greater than normal thickness. However, this leads to increases in the cost of production, which is especially undesirable in the manufacture of circuit boards having conductor lines consisting only of copper.
The aforementioned disadvantages of the prior art are avoided by the method of this invention in an economical and reliable manner. This invention also permits the use of an etching treatment which need not be as carefully controlled in comparison with prior art methods.